In many phases of manufacturing there is a need to activate a responsive device which will act on a moving object. In the packaging or product assembly phases of manufacturing, for example it is often desired to apply a bead of adhesive of a given length to a specific area of an object while the object moves on a conveyor past a dispensing device. Generally, the dispenser must be turned on and off at precise times in order to apply the adhesive to the proper area on the object. For ease of understanding the invention can be described in terms of this one specific application. Many other applications are possible.
In order to activate the dispenser in automated systems, a sensor is generally employed to detect articles moving on the conveyor. The sensor is generally located to sense the articles upstream from the dispenser. Therefore, the activation of the responsive device must be delayed for some period of time after an article is sensed, specifically, until the object reaches the dispenser. Thereafter, the dispenser is activated for some given duration of time, during which adhesive is applied to the object.
The amount of time for which the start of the activating control signal must be delayed and the duration of the activating signal are influenced by many factors, i.e.: conveyor speed; distance from the sensor to the dispenser; the distance between the triggering edge of the object and the location on the object where the bead is to start (for turn on) or bead length (for turn off); and the time required for the dispenser to turn on in response to a control signal (or drop out in response to removal of the control signal), or other system delays which are constant as a function of time irrespective of conveyor speed.
In an automated system wherein the conveyor always moves at the same speed, the amount of time which elapses between the time the object is sensed and the time the object is in proper position for dispensing adhesive is always the same. Also, the time for which the dispenser must be activated in order to apply a given length of an adhesive bead will again be constant. These times can be calculated. Therefore, it is a straightforward matter to simply delay activation and deactivation of the dispenser for a calculable time. Further, if there is a significant delay between the time an activating signal from a control circuit is generated, and the time a dispenser actually turns on (pull-in time) or turns off (drop-out time), these delays in dispenser actuation and deactuation can be included in the calculation to determine the precise time at which a control signal must be applied and removed from an automatic dispenser after a triggering event.
However, in many manufacturing facilities in operation today conveyor speed is variable within a fairly large range. Therefore, it is desirable to provide an automatic control circuit for actuation and deactuation of the dispensers wherein changes in line conveyor speed are compensated for automatically.
One approach to providing such a system is disclosed in U.S. Pat. No. 3,532,990. This patent discloses an analog system designed to automatically compensate for changes in conveyor speed as described above. That system is limited in many respects. Analog systems in general are prone to drift with time or temperature. Set up procedure, operating characteristics, and calibration procedures for that system appear to be very complex and susceptible to inaccuracies as far as calibration, set up and response to chamnges in system operating parameters.
Another system to solve the problem is disclosed in U.S. Pat. No. 3,813,524. That system is a digital system, and although its digital nature overcomes some problems associated with an analog system, it is a complex system which has a limited versatility. The system requires calculations in order to calibrate the system for any given installation, and further appears to be able to compensate only for the pull in time of the dispenser and not for the drop out time of a dispenser. The logic of the circuitry puts limitations on the operation of the system. The system cannot be retriggered until a complete dispensing cycle had been completed, that is until the gun is turned off. Further, the logic of the system necessitates circuit elements or functions not necessary in the system of the present invention described below.
All of the prior art systems were limited either in accuracy or versatility, or were overly complex.
It is an object of the present invention to provide an automatic control system which is more versatile, accurate and less complex than the prior art.
In the present invention, a digital circuit is provided which delays actuation or deactuation of the responsive device as a function of conveyor travel rather than time. Therefore, actuation/deactuation is delayed until the object is in proper position notwithstanding the time required for the object to arrive at this position. Thus, the actual time delay in actuation will be dependent on conveyor speed.
In one aspect of the preferred embodiment of the invention, the system comprises a photosensor, pulse tachometer, a distance or "lead length" counter, timer and control circuitry. The pulse tachometer generates one pulse for each predetermined increment of conveyor travel (each millimeter for example). In its application to automatic dispensing of adhesive onto moving objects, the counter is set to a predetermined count representing the distance between the leading triggering edge of an object to be treated and the point on the object where adhesive is to begin to be applied to the object, plus the distance between the photosensor and the dispenser, the total delay required being dependent upon the travel required for the object to travel from the sensor to the dispenser plus the distance from the triggering edge of the object where application is to begin. In other words, the counter is set to a number representing the travel of an object between its position where it is sensed and its proper position for the start of adhesive application.
With the counter being set to this predetermined number, a signal from the photosensor and control circuitry causes the counter to count pulses from the tachometer from this predetermined number to a preselected number. In the preferred embodiment the counter counts down to zero. Having the counter count to a preselected number, e.g. zero, reduces the complexity of the circuit and number of components. At a count of zero the conveyor will have moved the object to the proper position at which to treat the article. At the count of zero the counter generates a signal to start the responsive device.
A second distance counter, "bead length" or "duration" counter, can be provided to control the duration of the operation of the responsive device, such as to control the bead length of the adhesive applied to the object. In the example of an adhesive dispenser this second counter is preset to a predetermined count representing the bead length desired. A signal derived from the zero count of the first counter causes the second counter to count tachometer pulses until this second counter reaches a preselected count, preferably zero. At a zero count on the second counter the object has traveled the desired bead length and the responsive device or dispenser is deactivated and a dispensing cycle is completed. After the respective counters have reached zero, control circuitry presets the counters to their respective predetermined counts so that the counters are ready for another object.
As mentioned above, there are fixed time-dependent delays associated with almost all automatic systems. In the example of the adhesive dispenser the most notable fixed time delay characteristic of the system is caused by the mechanical devices; e.g. the time required for a dispenser to actually dispense after a control signal has been generated, or to stop dispensing after the control signal has been removed. In general these delays will be constant at any conveyor speed. Thus, it can be appreciated that, because of these fixed delays in response, the dispenser will not actually apply adhesive to the object until some fixed time after a control signal is generated. Because the delay is fixed as a function of time the conveyor travel will, for this fixed time, vary depending on the speed of the conveyor. At higher conveyor speeds this error can be significant.
It is an object of the system comprised of the present invention to provide a system which compensates for these fixed delays at any conveyor speed. According to the present invention it has been recognized that, because of these fixed delays, the control signal must be generated at an earlier time than would be required if no fixed delays were involved. The control signal must be generated earlier by a time equal to the delay which is affecting operation.
In the present invention compensation for fixed delay is effected by periodically presetting the distance counters to their predetermined counts and immediately causing the counters to count tachometer pulses for a time equal to the fixed delays associated with the function controlled by the counter. In other words, the count in the counter is reduced by the number of pulses occurring within the period of the fixed delays. Therefore, when counting in response to a sensed object the counter will reach the preselected count (zero) earlier in time by an amount equal to the fixed delays associated with the function controlled by the counter, and at any conveyor speed. Therefore, the counters provide a control signal to the responsive device at a time which allows the responsive device to actually initiate its function when the object is at the proper position.
This periodic updating is repeated until an object is sensed by the photosensor. Thus, the conveyor speed for which compensation has been made is sensed very close in time to any activation of the control system by the photosensor. The time between periodic updates will depend on how fast conveyor speed can change, and therefore may be different in different applications.
Each travel related counter is individually compensated for fixed time delays associated with the counter. Independent compensation can be provided for both pull-in and drop-out fixed time delays. For example, in the presently considered application to an adhesive dispenser, the lead length counter is compensated for pull-in delays and the bead length counter is compensated for drop-out delays.
Thumb wheel switches having coded outputs compatible with the counters can be used to adjustably set the counters to their predetermined counts. The system is calibrated so that operating adjustments can be set in standard units of measure, e.g. milliseconds and millimeters. When used with an adhesive dispensing apparatus, the desired bead length of adhesive can be dialed in directly in millimeters, for example. The distance between the dispenser and sensor is added to the distance behind the leading triggering edge of the object at which the bead is supposed to start and this total distance is dialed directly into the system in standard units of measure. The bead length is likewise dialed in in standard units of measure. The compensation timers are then adjusted to the proper duration to correspond to pull-in time and drop-out time of the responsive device.